In recent years, in the automobile industry, metal pipe is increasingly being used as one means for reducing weight. Hollow metal pipe, compared with a solid material, offers the same rigidity while enabling the cross-sectional area to be reduced.
Further, an integral structure of metal pipe, compared with a T-shaped structure obtained by welding two metal plates, enables a reduction of weight by the elimination of the need for a welded flange part.
However, auto parts are placed in narrow spaces in the automobiles. Therefore, metal pipe is seldom used as is as a straight pipe. It is almost always attached after being secondarily worked.
As secondary working, bending is used most often, but in recent years the increasing complexity of the shapes of auto parts has led to an increase in hydroforming (fastening a metal pipe in a mold and, in that state, using inside pressure and axial direction compression to work the pipe into the mold shape) as well and, further, an increase in working comprised of these working processes overlaid.
Hydroforming itself, as shown in FIG. 1 (see Sosei [Plasticity] and Kakou [Working], Vol. 45, No. 524 [2004], p. 715), compared with the simple T-forming, is being used for increasingly complex shapes in recent years. The pipe expansion rates (ratio of circumferential length of product pipe to circumferential length of stock pipe) have also been increasing.
Japanese Patent Publication (A) No. 2002-100318 discloses the method of obtaining a straight shaped hydroformed product with a large pipe expansion rate. However, this method is a method of expanding a metal pipe in one direction, then expanding it in a direction at right angles to this direction. It cannot be applied to working including bending.
Japanese Patent Publication (A) No. 2002-153917 and Japanese Patent Publication (A) No. 2006-006693 disclose a method of obtaining a worked part of a shape including bending and with a large pipe expansion rate.
The method described in Japanese Patent Publication (A) No. 2002-153917 is a method of using a movable mold to obtain a hydroformed product having a high branch pipe height. This can be used for cases of expansion in only one direction like with forming a branch pipe. Expansion in a direction at a right angle to that is not possible.
The method described in Japanese Patent Publication (A) No. 2006-006693 is a method of hydroforming, then rotary draw bending, but this method also can be applied to the case of expansion in only one direction. It does not enable expansion in a direction at a right angle to that.
The method described in Japanese Patent Publication (A) No. 8-192238 is the method of forming a hydroformed part with a bent part expanded in one direction and a direction vertical to that.
In this method, at a first step, the pipe is expanded concentrically, so even in the final shape, the result is a shape expanded to both of the outer side and inner side of the bend.
However, as explained above, auto parts have to be arranged in narrow spaces inside an automobile. It is not always necessary to expand the pipe to the outer side and inner side of the bend.
Conversely, to avoid interference with other parts, sometimes the inner side of the bend is hard to expand or the outer side of the bend is hard to expand. In such a case, the method described in Japanese Patent Publication (A) No. 8-192238 cannot be applied.
That is, in the conventional hydroforming method, when trying to increase the pipe expansion rate of the bent part, either the pipe was expanded in just one direction or the pipe was expanded across the entire circumference including the outer side and inner side of the bend.
For this reason, inherently, there was the problem that it was not possible to use hydroformed parts, which contribute to lighter weight, for suspension parts and other structural members of automobiles arranged in restricted spaces.
Further, plastic working other than hydroforming, for example, bending, cannot be used to work a structural member of the above shape. If using casting, it is possible to obtain a structural member of the above-mentioned shape, but a cast part is inferior to a plastic worked part in toughness, weldability, etc., so cannot be used for a suspension part, body part, or other auto part mentioned at the start.
In addition, it is possible to provide a heat treatment step during the bending or hydroforming to repeatedly eliminate the work strain caused in the previous step and finally obtain a greatly deformed worked part.
However, if providing a heat treatment step, (a) the production costs rise, (b) the production efficiency falls, (c) a dedicated facility for heat treatment of the complicatedly shaped part after working becomes necessary, (d) the heat treatment causes formation of scale at the surface of the part, (e) the effect of work hardening is reduced, etc. Numerous disadvantages accrue.